THE ESTIMATION OF METABOLISM 171 



Such an amount of protein must have yielded in its falling 

 apart a quantity of carbon represented by the percentage 

 of that element in the compound. The actual percentage 

 is about 53, so that in this instance 53 grams of the 225 

 may be ascribed to protein as a source. The remaining 

 carbon, 172 grams, must have been derived from non- 

 nitrogenous material. How far it has come from fat and 

 how far from carbohydrate we cannot exactly determine 

 without additional data. 



It is of value to know all the circumstances of such a 

 trial. If the twenty-four hours under consideration is the 

 first fasting day and the diet of the day before has been the 

 ordinary one, we may assume that the subject entered 

 upon the experimental period with a fair stock of glycogen. 

 This will be used rather freely at the outset, but more 

 and more slowly as the hours pass. Carbohydrate, accord- 

 ingly, contributes largely to the support of the organism 

 during the first day of abstinence, and thereafter bears but 

 a very small part. To say that the glycogen of the body 

 is used up in a single day of fasting would not be correct; 

 the fact is rather that the rate of consumption diminishes 

 sharply. In proportion to this diminution in the use of 

 carbohydrate the fat is called upon increasingly. For any 

 day of hunger after the first it is substantially true that 

 the individual is living on protein and fat. 



Let us continue the discussion of our numerical illus- 

 tration with the added statement that the day is the second 

 rather than the first in a fast. The 172 grams of carbon 

 from non-protein material may now be attributed to fat. 

 The percentage of carbon in fat is about 77. A simple cal- 

 culation (dividing by 77 and multiplying by 100, or, what 

 is the same thing, multiplying our first quantity by 1.3) 

 gives 223.6 grams of fat as the amount destroyed in the 

 body during twenty-four hours. The total metabolism 

 is then 100 grams of protein and 223.6 grams of fat. It is 

 not safe to conclude from this that the loss of weight 

 will prove to be j ust equal to the sum of the two items. It 

 may be found to be either more or less, the result depend- 



